European leaders Saturday held telephone talks with Russian President Vladimir Putin and his Ukrainian counterpart after a Ukrainian military plane was shot down by separatists , killing all 49 people on board , officials said .

The Ilyushin-76 military transport plane went down early Saturday while approaching an airport in the eastern Ukrainian city of Luhansk , said military spokesman Vladislav Seleznyov from the anti-terror operation unit .

Ukraine 's defense ministry said the plane , which was transporting military personnel , was shot down by insurgents using anti-aircraft machine guns .

The attack , which represents one of the bloodiest single events in that nation 's current period of turmoil , spurred a diplomatic flurry to limit further escalation of the crisis .

On Saturday , French President Francois Hollande and German Chancellor Angela Merkel held a telephone discussion with Putin on the crisis in Ukraine , according to the Elysee Palace and the Kremlin .

Hollande and Merkel expressed their deep concern about the ongoing fighting in eastern Ukraine , particularly Saturday 's attack , the Elysee Palace said . The French and German leaders stressed the importance of reaching a cease-fire in Ukraine and avoiding the transfer of fighters and weapons across the Ukrainian-Russian border , according to Hollande 's office .

The Kremlin said the three leaders also talked about the drafting of a United Nations Security Council resolution aimed at promoting a settlement . The European leaders stressed the need for an agreement on continuing Russian natural gas deliveries to Ukraine , according to the Elysee .

`` Situation is dangerous , but I hope will remain under control , '' Ukrainian Deputy Foreign Minister Danylo Lubkivsky told CNN Saturday . `` With no doubt , more pressure on Russia is needed . ''

In a phone conversation with Hollande , Ukrainian President Petro Poroshenko urged the European Union to immediately consider sanctions against Russia , including the termination of `` military-technical cooperation , '' Poroshenko 's office said .

Ukrainian Prime Minister Arseniy Yatsenyuk spoke by phone with U.S. Secretary of State John Kerry , who expressed condolences for the loss of life and `` strong concern '' about the flow of heavy weapons and militants across the border from Russia , a senior State Department official said .

Kerry also talked with Russian Foreign Minister Sergey Lavrov , voicing his concern over the downing of the Ukrainian transport plane and pressing Lavrov to `` make clear '' Russia 's commitment to peace , a cease-fire and political dialogue , the state department official said .

In a statement , Poroshenko said the army and state border service had closed 248 kilometers of Ukraine 's border .

`` The goal us to take back control of Ukraine over the state border and provide adequate response to terrorists , '' he said .

A U.S. National Security Council spokeswoman Saturday condemned that attack , saying officials were `` deeply concerned about the situation in eastern Ukraine , including by the fact that militant and separatist groups have received heavy weapons from Russia , including tanks , which is a significant escalation . ''

`` Regarding sanctions , we have said that we want to give Russia the opportunity to pursue de-escalation , '' NSC spokeswoman Laura Lucas Magnuson said in the statement . `` President Obama made clear that de-escalation depends upon Russia ceasing support for separatists in eastern Ukraine and stopping the provision of arms and materiel across the border , and we urge Russia to do so . ''

Video posted to YouTube claimed to show the plane being shot down . Small flashes can be seen climbing skyward , followed by large flashes on the ground on the distant horizon .

The plane was also carrying military machinery and supplies , the ministry said . Officials originally had said the incident occurred late Friday but later updated the time .

Poroshenko announced a day of mourning Sunday to honor those killed and said Ukrainians are grieving .

`` It is a great loss not only for the families of the deceased ones , but for the whole country , '' he said . `` Everyone involved in this cynical , large-scale terrorist act will be punished . ''

In an indication of the continued opposition from some quarters to the Kiev government , the state security office said in a statement Saturday that a homemade bomb was found overnight outside a gate to Poroshenko 's office .

The device was made of grenades and a kilogram of other material , such as metal screws , and had a threatening note attached that demanded that the President stop military action against the separatists , the statement said . The person who left it was spotted but fled without detonating it .

The Kiev-based government is carrying out what it calls an anti-terrorist operation , centered in the Donetsk and Luhansk regions , to try to regain control from pro-Russia separatists .

Russia and Ukraine have been engaged in a tense standoff since March , when Russia annexed Crimea and massed troops along its border with Ukraine . Moscow has since begun to withdraw those forces .

NATO : Tank images raise questions

NATO released images Saturday that it said raised `` significant questions concerning Russia 's ... involvement in the movement of military equipment from Russian territory into Ukraine . ''

Ukraine 's acting Interior Minister Arsen Avakov claimed Thursday that three Russian tanks had crossed the border into Ukraine as part of a larger armored column , prompting a skirmish between Ukrainian and Russian forces . The vehicles entered at a checkpoint controlled by separatists in Ukraine 's eastern Luhansk region , Avakov said , citing Ukrainian intelligence .

The Russian Foreign Ministry told the BBC on Thursday that the claim its tanks had crossed the border was `` another fake piece of information . ''

But the United States also believes tanks and heavy weapons have crossed into Ukraine from Russia after moving from a deployment site in southwest Russia , State Department spokeswoman Marie Harf said Friday .

The images released by NATO show the movements of Russian forces at a site near Russia 's southwestern city of Rostov-on-Don , about 75 kilometers from a border crossing the body says is controlled by Luhansk separatists .

No tanks were at the site on May 30 , according to NATO . An image from June 6 shows the departure of the Russian military unit but the arrival of 8 main battle tanks . On June 11 , 10 main battle tanks can be seen at the site , NATO said . Of these , three have been loaded onto the kind of low trucks `` normally used to move tanks , likely indicating imminent movement by road . ''

NATO also cites video posted online showing tanks on the move in Makiivka and Snizhne in eastern Ukraine , in one instance accompanied by a vehicle flying a Russian flag .

`` The tanks do not bear markings or camouflage paint like those used by the Ukrainian military . In fact , they do not have markings at all , which is reminiscent of tactics used by Russian elements that were involved in destabilizing Crimea , '' the NATO statement said .

It also accused Russian officials of having been `` repeatedly misleading and evasive regarding their roles in both Crimea and eastern Ukraine . ''

If the latest reports are confirmed , the statement added , `` this would mark a grave escalation of the crisis in eastern Ukraine in violation of Russia 's Geneva commitments . ''

U.S. and EU leaders have warned that additional economic sanctions could be imposed on Russia if it further escalates its incursion into Ukraine .

Russian leaders in turn say Ukraine has failed to move to implement the framework for peace worked out in Geneva , Switzerland , in April . They say Ukrainian leaders have carried out a campaign of violence against people living in the largely pro-Russian east .

Mariupol operation

On Friday , the Kiev government announced that an operation targeting pro-Russian separatists in the city of southeastern port city of Mariupol had been successful .

Anton Geraschenko , adviser to Avakov , said more than 30 `` terrorists '' had been detained and their base destroyed . Other separatists are hiding in homes and basements for safety , he said .

Four Ukrainian soldiers were injured in the operation , he said .

Residents of the city may in the past have blocked Ukraine 's security forces , he said , but `` today , not one resident of Mariupol protected terrorists . ''

According to the Interior Ministry , the yellow-and-blue Ukrainian flag is flying once again over the main municipal building in Mariupol . The city has changed hands several times in the course of hostilities over the past several months .

But the unrest has n't ended .

Pro-Russian separatists wielding automatic weapons and grenades attacked Ukraine border service vehicles Saturday near the city , Ukraine 's State Border Service said . Five people were killed and seven others were wounded , the service said . The self-proclaimed Republic of Lugansk took credit for the attack , service spokesman Oleg Slobodyan said Saturday .

Diplomatic moves

Western leaders have accused Moscow of fomenting instability in eastern Ukraine and have urged Moscow to engage with the new leadership in Kiev .

On Thursday , Poroshenko and Putin `` held a substantial and long phone conversation , '' the Ukrainian President 's media office said . The leaders discussed Poroshenko 's peace plan to resolve the situation in the east of Ukraine , it said .

Poroshenko has called on the rebels to lay down their arms and engage in talks .

Also Thursday , Vitaly Churkin , Russia 's U.N. ambassador , said he would introduce a resolution on Ukraine at the U.N. Security Council in light of what he said was a deteriorating situation in the country .

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NATO releases images showing tank presence near Russia 's border with Ukraine

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Security service says a homemade bomb was left outside gate of President 's offices

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Three Ukrainian soldiers killed in attack by separatists

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Ukraine 's President vows punishment for plane shootdown